In conventional pattern tracing systems of the optical type, a scanner mirror forms a part of an optical system which normally projects an image of a photocell onto a line or an edge that is part of the pattern to be traced. When the scanned image of the photocell traverses the pattern, signals are obtained from the photocell which are used to derive directional and displacement signals which in turn are used to derive signals for x and y coordinate servo-systems that maintain the optical scanning system positioned properly above the pattern while driving it along the pattern at a preselected speed.
The feed speed signals for the tracing head are generally set to a predetermined value on a control panel, which value when multiplied by the sine and cosine of the angle that the pattern makes with a reference direction form individual coordinate feed signals which are usually directly added into the servo input signals. Sine and consine signals are derived from the optical scanning information, either with electromechanical syncro resolvers in the tracing head or with electronic sine/cosine resolving circuits, and these signals are fed to the respective servo-motors to control servo-motor speed along each coordinate to that the tracing head follows the pattern to be traced.
Optical pattern tracers of this type are widely used in the industry to guide machine tools such as flame cutting machines or milling machines along a path identical to a flat or two-dimensional pattern. Examples of these systems are illustrated in the Barry et al U.S. Pat. Nos. 2,499,178, the Brouwer 3,017,552 and the Jewel 3,322,952.
In most of these systems, the tracing head or a scanning element in the tracing head is pivotally mounted and has a steering servo which pivots the scanning element about a reference axis approximately perpendicular to the pattern segment being scanned. Many of these systems mount the scanner on an arm the radius of which may be manually adjusted. This arm is maintained perpendicular to the pattern by the servo-motor and it compensates for errors caused by the radius of the cutting tool. This is sometimes referred to as "kerf offset" and is equal to the effective tool radius or half the width of the flame that cuts the path in a flame cutting tool. This method is effective and works well as long as the steering servo has a sufficiently fast response to cope with the required rate of rotation when the machine moves around the corners of the pattern.
These systems have the primary disadvantage that the cost of the steering servo and the mechanical pivot assembly associated therewith for the scanning element significantly add to the cost of the tracing system. The steering servo systems have the additional disadvantage of requiring the forward and kerf offsets to be primarily mechanical manipulative adjustments. The steering servo may be eliminated but by so doing the tool radius compensation feature, or "kerf offset" is manually also eliminated, and systems representing this type are disclosed in the Parker et al U.S. Pat. No. 3,704,372 and the Hannappel et al U.S. Pat. No. 3,534,162. The Hannappel et al system, for example, includes an annular array of fixed photosensitive elements which are electronically sequentially scanned. However, this system has no compensation for the full tool radius.
In addition to not providing full tool radius compensation, the systems that eliminate the steering servo for the scanner, also have very limited control over the forward offset of the scan circle. The forward offset is generally required for control stability at higher speeds, since the effects of mechanical inertia of the parts increase geometrically with the speed of the scanner frame and the moving parts carried thereby. In prior scanning systems which do not have the steering servo for the scanning head, the forward offset is effectively equal to the scanning circle radius which cannot easily be changed or varied. Therefore, since a larger forward offset is generally needed for control stability at higher speeds, these systems usually compromise by using a forward offset, or scanning circle radius, much larger than needed at slower machine speeds. This sacrifices low speed accuracy and consequently larger geometric errors are caused than desired at lower speeds by such tracing systems.
Still other tracing systems employ a continuously rotating scanner motor instead of a steering servo, and in these a reference point is defined by the intersection of the axis of rotation of the scanning motor with the pattern. In this system the optical axis of a photocell intersects the pattern a distance away from the reference point and describes a circle around the reference point. A photocell signal is obtained when this scanning circle intersects the pattern which occurs twice per revolution, although only the forward intersection point is normally employed to develop control signals. The forward offset is equal to the radius of the scanning circle since the photocell looks at a pattern point separated by distance R from the reference point. In such a system R, or the forward offset, can only be changed mechanically such as by using a set of mirrors having a different tilt selectively mounted by the operator on the scanning motor shaft. This is an extremely cumbersome system and very difficult for the operator to manipulate with any degree of efficiency.
The side offset, or kerf compensation, is even more difficult and limited in systems having the continuous rotary scanner motor-mirror. Since the direction of the pattern is measured by the phase of the photocell pattern crossing-time relative to the motor driving sine wave, by adding or subtracting an additional or fixed phase shift, the crossing point we look at is not necessarily in the forward direction but may be at an angle away from the forward offset direction. In these systems, the kerf offset cannot be even as large as the radius R of the scan circle, since forward offset would be zero (if kerf offset equals the radius of scan) and in this case the directional sense of the system is lost.
Since a larger forward offset is needed at higher speeds, and a lower forward offset is desirable at lower speeds, even the most flexible of these prior systems requires the operator to shut down the tracer and mechanically adjust the forward offset by one of a variety of methods described above when changing the tracing speed.